Compositions comprising metallic substrates having deliberately (also referred to as pre-oxidized) oxidized or nitrided surfaces have many industrial, medical, and other applications. The use of such surfaces results in modified behavior of the interfacial surface regions to optimize their interaction with other materials. The manufacture of such compositions, like most manufacturing processes, typically results in yields of less than 100%. There is a need for improvement in the efficiency of the manufacture of such compositions, and this need is particularly acute where the cost of the substrate is high and the resulting costs of scrapping non-conforming material is high.
One field that has benefited from the use of compositions comprising metallic substrates having pre-oxidized or nitrided surfaces is the field of medical implants. Medical implant materials, in particular orthopedic implant materials, must combine high strength, corrosion resistance and tissue compatibility. The longevity of the implant is of prime importance especially if the recipient of the implant is relatively young because it is desirable that the implant function for the complete lifetime of a patient. Because certain metal alloys have the required mechanical strength and biocompatibility, they are ideal candidates for the fabrication of prostheses. These alloys include 316L stainless steel, chrome-cobalt-molybdenum alloys, titanium alloys and more recently zirconium alloys which have proven to be the most suitable materials for the fabrication of load-bearing prostheses.
To this end, oxidized zirconium orthopedic implants have been shown to reduce polyethylene wear significantly. The use of diffusion-hardened oxide surfaces such as oxidized zirconium in orthopedic applications was first demonstrated by Davidson in U.S. Pat. No. 5,037,438. Previous attempts have been made to produce oxidized zirconium layers on zirconium parts for the purpose of increasing their abrasion resistance. One such process is disclosed in U.S. Pat. No. 3,615,885 to Watson which discloses a procedure for developing thick (up to 0.23 mm) oxide layers on Zircaloy 2 and Zircaloy 4. However, this procedure results in significant dimensional changes especially for parts having a thickness below about 5 mm, and the oxide film produced does not exhibit especially high abrasion resistance.
U.S. Pat. No. 2,987,352 to Watson discloses a method of producing a blue-black oxide layer on zirconium alloy parts for the purpose of increasing their abrasion resistance. Both U.S. Pat. No. 2,987,352 and U.S. Pat. No. 3,615,885 produce a zirconium dioxide layer on zirconium alloy by means of air oxidation. U.S. Pat. No. 3,615,885 continues the air oxidation long enough to produce a beige layer of greater thickness than the blue-black layer of U.S. Pat. No. 2,987,352. This beige layer does not have the wear resistance of the blue-black layer and is thus not applicable to many parts where there are two work faces in close proximity. The beige layer wears down more quickly than the blue-black oxide layer with the resulting formation of oxidized zirconium particles and the loss of the integrity of the oxidized zirconium surface. With the loss of the oxide surface the zirconium metal is then exposed to its environment and can lead to transport of zirconium ions away from the surface of the metal into the adjacent environment.
The blue-black layers have a thickness which is less than that of the beige layer although the hardness of the blue-black layer is higher than that of the beige layer. This harder blue-black oxide layer lends itself better to surfaces such as prosthetic devices. Although the blue-black layer is more abrasion resistant than the beige layer it is a relatively thin layer. It is therefore desirable to produce the blue-black layers of increased abrasion resistance without producing the same type layers of the prior art.
As discussed above, U.S. Pat. No. 5,037,438 to Davidson discloses a method of producing zirconium alloy prostheses with a blue or blue-black oxidized zirconium surface. The prostheses of Davidson '438 exhibited exceptional wear characteristics. In U.S. Pat. No. 5,180,394, Davidson suggested the use of nitrided surfaces of zirconium or zirconium alloys. U.S. Pat. No. 2,987,352 to Watson discloses a method of producing zirconium bearings with a oxidized zirconium surface. The oxide layer produced is not always uniform in thickness and the non-uniformity reduces the integrity of the bonding between the zirconium alloy and the oxide layer and the integrity of the bonding within the oxide layer. Both U.S. Pat. No. 2,987,352 and U.S. Pat. No. 5,037,438 are incorporated by reference as though fully set forth herein.
In U.S. Pat. Nos. 6,447,550; 6,585,772 and pending U.S. application Ser. No. 10/942,464, Hunter, et al. described methods for obtaining an oxidized zirconium layer of uniform thickness. Hunter teaches that such is obtained by applying pre-oxidation treatment techniques and by manipulation of substrate microstructure. The use of uniform thickness oxide layer results in increased resistance to corrosion by the action of the body fluids as well as other benefits and is biocompatible and stable over the lifetime of the recipient. U.S. Pat. Nos. 6,447,550; 6,585,772 and pending U.S. application Ser. No. 10/942,464 are incorporated by reference as though fully set forth herein. In another approach of the prior art, Treco (R. Treco, J. Electrochem. Soc., Vol. 109, p. 208, 1962) used vacuum annealing method to dissolve the oxide formed on Zircalloy-2 after corrosion testing. The objective of Treco's work was to partially eliminate the oxide by vacuum annealing and then remove the hardened zone by acid pickling. Treco did not want to re-oxidize the samples hence the dissolution of oxide in the substrate and its influence on the re-oxidation was not considered.
The diffusion-hardened surfaces of Davidson and Hunter, while having relatively thick ceramic oxide or nitride layers, did not exhibit thick diffusion hardened zones below the ceramic oxide or nitride. The diffusion hardened zones of the compositions of Davidson and Hunter had thicknesses of at most 1-2 microns and typically less depending upon the conditions used to produce the composition. While the resulting compositions of Davidson and Hunter exhibited high wear resistance in comparison to those compositions available in the prior art, there is still room for improvement.
This significant reduction in wear in oxidized surfaces is attributed to its ceramic nature of the surface. The oxidized zirconium implant typically has 5 to 6 micron thick ceramic surface (zirconium oxide) that is formed by a thermally driven diffusion process in air. Beneath the zirconium oxide is a hard, oxygen-rich diffusion layer of approximately 1 to 2 micron. The totality of hardened zones (oxide plus diffusion hardened alloy) render the implant resistant to microscopic abrasion (third bodies such as bone cement, bone chips, metal debris, etc.) and slightly less resistant to macroscopic impact (surgical instrumentation and from dislocation/subluxation contact with metallic acetabular shells). The relatively small thickness of the hardened zones in the prior art oxidized zirconium compositions make them susceptible to damage caused by dislocation and subluxation. Thus, while the application of diffusion-hardened oxide layers such as oxidized zirconium to orthopedic implants has resulted in improvements in abrasion resistance and service life, there is room for improvement.
While the benefits to the use of oxidized zirconium, as well as other oxidized or nitrided compositions, are now well-known, improvements in the manufacture of such products are needed. One of the drawbacks of such products is that after oxidation or nitridation, if the parts do not meet specification, they are scrapped. The rework requires removal of the oxide by mechanical or chemical machining/polishing. Mechanical machining can lead to oxide particles embedment that can interfere with re-oxidation and/or re-nitridation. The re-work refers to a method or process applied on a part that does not meet the specification. The re-work dissolves or eliminates the oxide/nitride of the oxidized/nitrided surface. After rework, the components can be re-oxidized or re-nitrided (i.e., the oxide and/or nitride layers may be reformed after removal of the original oxidation and/or nitridation layers in the rework procedure). The removal of oxide by chemical means can alter the substrate surface and hence the re-oxidation characteristics. These rework techniques further more result in dimensional changes of the components. The inventors herein describe a process to rework oxidized or nitrided compositions to improve yield and manufacturing efficiency, thereby lowering the cost of products comprising oxidized or nitrided compositions, including medical implants comprising oxidized zirconium.
All of the above-referenced U.S. patents and published U.S. patent applications are incorporated by reference as though fully described herein.